Yeah, 7500 total for three people will be fine for a housing allowance in Beijing. 8000 is on the low end for Beijing, especially assuming that you'll be teaching 20+ hours a week, as is typical for BJ. That said, there are two of you and you won't starve on that kind of money, it just isn't really that great of a deal.
The only thing I'd be extra cautious about is whether or not they're getting you a Z visa and then a residence permit. Like the others said, the shady schools will lie to get you in the door. If they say come over on an L, we'll change it when you get here, don't take the offer. If they say it is legal to work on an F visa, don't take the offer. With your issue of not having a degree, you need to be extra careful because Chinese law requires a degree (regardless of experience -- there may be some exceptions for certain kinds of experts, but not teachers afaik), so most of the legitimate decent schools won't be able to hire you.